This invention relates generally to valve apparatus for controlling fluid flow and more particularly to such apparatus for controlling the flow of hydraulic fluid in an automotive automatic transmission system.
A conventional transmission system includes two grooved plates positioned against opposite sides of a spacer member. The grooves in the plates match up and are interconnected by apertures in the spacer member to form a selected pattern of fluidic control passageways and chambers. When an automatic transmission is shifted, fluid flows in certain passageways to effect shifting of the transmission. Control orifices of specific size depending on the viscosity of the fluid and pressure to which it is subjected are employed to provide a smooth shifting function however it has been found that as the transmission fluid warms up and becomes less viscous, the fluid flow becomes excessive when shifting from park to first or from neutral to reverse. In order to compensate for this, it has been proposed to provide a temperature responsive valve such as one controlled by a thermostat metal element which would regulate the size of a control orifice between a maximum orifice diameter to assure adequate flow at a selected low temperature and a minimum orifice diameter at temperatures above the temperature at which further viscosity changes of the fluid are no longer significant. In one application a maximum diameter of 0.120 inch at 75.degree. F. and a minimum diameter of 0.090 inch at 100.degree. F. and above were selected. With adequate space a thermostat coil could provide sufficient movement to modulate the size of an orifice between these two limits however one of the constraints in providing a valve for a transmission system is the limited spaced available for placement of the valve, for example in the order of 10 mm.times.10 mm.times.10 mm. This constraint is exacerbated by the need to provide sufficient overtravel for the thermostat metal element so that it will not be limited in its movement by a stop which would induce undesirable stresses in the thermostat metal element thereby changing its calibration and reducing its useful life.